The invention relates to a method for detecting and monitoring the temperature of the winding of a coil, preferentially the magnet coil of a magnetic valve. The invention furthermore concerns a circuit suitable for the practice of this method.
The size of coils, whether for electric motors, magnetic valves, transformers or the like, depends on the current flowing through the winding. If the current exceeds for any length of time an allowable limit, the heating of the winding can result in insulation damage. To prevent such unacceptably high heating of a coil, it is known (cf. German Federal publication OS No. 25 36 375) to sense the coil temperature by means of a temperature sensor, which simultaneously serves a safety function.
In the case of coils which are subject to heavy loading it is known to operate them briefly with an excess excitation in order to be able to avoid unnecessarily large windings. This technique is known especially in the case of magnetic valves which briefly require a high excitation current during a "pull-in" phase, while the rest of the time only a so-called holding current is required, which holds the valve plunger in a certain position, e.g., against the force of a spring. The over-excitation during the operating phase serves as a rule to permit more force to be available for the acceleration of the valve plunger than is needed for holding it in a particular position.
Frequently it is necessary to overcome as quickly as possible any load-caused sticking or other forces which may be caused by an unfavorable operating-force characteristic of the driving magnet. The operation of a magnetic valve at briefly elevated excitation leads to an especially economical design of the magnet, since its size can be governed mostly by the holding excitation and the necessary number of operating cycles per unit of time, not by the pull-in excitation alone.
It is the object of the present invention to propose a method and a circuit of the above kind, which will be especially simple and will not require additional sensing means such as a temperature sensor, for example.